Rotary sprinklers are well known having a gear drive assembly powered by the force of the water passing through the sprinkler. The drive assembly typically includes a drive housing which encloses a planetary gear train. A water driven turbine located adjacent one end of the drive housing drives a turbine shaft that rotates the gear train within the drive housing. An output shaft at the other end of the drive housing is driven by the gear train and is attached to the nozzle assembly for rotating the nozzle assembly at a speed slower than the speed of rotation of the turbine.
In a drive assembly of this type, a plurality of planetary gear stages are used. The total speed reduction provided by the drive assembly depends upon the number of planetary stages that are used and the amount of speed reduction provided by each stage. It is common for each stage to include a plurality of planetary gears that engage an internal ring gear on the inner diameter of the drive housing. For the drive assembly to work properly, the planetary gears have to be properly aligned prior to the insertion of each stage into the drive housing so as to properly engage the teeth of the internal ring gear. If they do not, the gears will bind or otherwise malfunction and the drive assembly will not operate properly.
In one known manufacturing method for this type of drive assembly, the planetary gear stages are assembled onto the spindle of an assembly fixture until all of the planetary gear stages are stacked on top of one another. Then, an assembler inserts this stacked subassembly by hand into the drive housing. During this process, the assembler has to carefully observe the planetary gears in each planetary gear stage as each stage drops down into the housing to make sure that the planetary gears properly engage the grooves in the ring gear. If they are not so positioned, the assembler has to slightly rotate one or the other of the planetary gears using his or her finger to attempt to ensure the necessary alignment.
The need for visual observation of each planetary gear stage as it is assembled in the drive housing, and the need to often manually rotate or jiggle the planetary gears so that they engage the ring gear without binding, makes it difficult to assemble such drive assemblies. It requires a skilled assembler who is paying careful attention to the assembly process. Even so and considering the small size of the gears involved in this type of drive assembly, it is not unusual for at least some of the drive assemblies to be misassembled and rendered useless. The manufacturing technique just described has a scrap rate on the order of 4% or so even when the drive assemblies are being assembled by skilled and attentive assemblers.